The invention is broadly directed to optical transmission systems that utilise optical amplifiers, and specifically to wavelength division multiplexed (WDM) systems. The invention has particular relevance to the control of output power of optical amplifiers in the presence of amplified spontaneous emission (ASE).
In any optical network it is important to maintain correct power levels for all traffic channels. This is generally achieved by monitoring the output power of optical amplifiers using a broadband optical detector, such as a photodetector. The monitored output power is then utilised in a feedback loop to adjust the amplifier gain so that the desired output is produced. The amplifier output power Pout is then compared with a desired, or target, signal power Poutxe2x80x94target. This target power Poutxe2x80x94target is the total power of all channels transmitted in the optical fibre. It can thus be expressed as
Poutxe2x80x94target=Nchxc3x97Pchxe2x80x94target
where Nch is the number of channels carried in the WDM link.
However, a characteristic of optical amplifiers, whether they are of the active fibre, semiconductor or solid state type, is amplified spontaneous emission (ASE) which manifests itself as a broadband signal at the amplifier output. For high input signal powers, the measurable ASE power at the output is negligible. However, at low signal powers, for example powers lower than about xe2x88x9220 dBm, the power due ASE is a significant proportion of the total measured output power. If the signal output power is corrected by adjusting the amplifier gain on the basis of the target power Poutxe2x80x94target, the resultant channel output power will inevitably be lower than required.
It is known to utilise a narrow band detector at the output of the amplifier to measure the signal power at a limited range of wavelengths. This effectively filters out the ASE so that the monitored signal is a faithful copy of the output traffic power. Such a solution may be used in systems using a single carrier wavelength, such as time domain multiplexed (TDM) systems, however, it is not so effective for WDM systems where a large number of different wavelengths are used. Narrow band detection may be employed for one of the signal wavelengths present in the WDM system but is problematic for two reasons. Firstly, the system becomes inflexible, since the monitored signal must be routed through all the optical amplifiers in the network. Secondly, the system is inherently frail because any fault occurring in the monitored channel will result in the collapse of the whole network.
Co-pending European patent application No. 99118310.4 suggests a numerical method for determining the generated ASE power perceived at the output of an optical amplifier. In this method all input power is treated as traffic signal power. Thus while the method is effective for determining how much ASE power is added to the total power output by an optical amplifier, it does not allow stabilisation of the output traffic power in the presence of propagated ASE power, i.e. accumulated ASE power generated by upstream optical amplifiers and contained in the input signal.
It is an object of the present invention to provide an optical amplifier arrangement and a method which enables the traffic signal power in an output signal to be reliably monitored.
It is a further object of the present invention to provide an optical amplifier arrangement and a method with which the output power can be reliably stabilised.
It is yet a further object of the present invention to provide an arrangement and method for an optical communications link comprising multiple optical amplifiers with which the traffic power may be reliably monitored and/or stabilised.
In accordance with the present invention, an optical amplifier receives information concerning the amount of ASE noise power contained in the amplifier input. This information may be transmitted from an upstream amplifier, or other device that generates ASE, of from a network controller located at one or both ends of the link or at one or more nodes. The amplifier is further provided with a control unit for determining the ASE power generated within the amplifier. This is preferably achieved by using a stored relationship between generated ASE power and gain of the amplifier. The total ASE power at the output is a combination of the generated ASE power and the amplified received ASE power. The total ASE power is communicated to any optical amplifier arranged immediately downstream as a fraction of the total output power, so that the same calculation may be made. Once the amount of ASE noise power is known, the actual traffic power at the amplifier output can be deduced, and the amplifier gain may be adjusted to stabilise this output power.